In CDMA communications systems, maximum uplink capacity is achieved when the power level of signals received by the Base Transceiver Subsystem (BTS) is the same for all mobile users. Such a power level is called the "nominal power level." If this nominal power level is maintained regardless of the distance between the base station and the mobile unit and regardless of the signal propagation environment then maximum uplink capacity is maintained.
If the power level of a signal received from a mobile unit drops below the nominal level, the error probability for that user increases. If the power level of a signal received from a mobile unit exceeds the nominal level, the probability that the signal will interfere with signals from other mobile units increases. Thus transmission power deviations from the nominal power level decrease the capacity of the system.
Rayleigh fading is a problem which introduces a fast power deviation from the nominal power level and thus degrades system capacity.
In current Third Generation Partner Project (3GPP) systems, power control mechanisms are being employed which attempt to equalize the received power of the signal from the mobile unit and to compensate for fast power deviations from the nominal power level caused by the impact of Rayleigh fading.
The overall uplink power control for 3GPP is shown in FIG. 1. The BTS employs a Rake receiver to receive and demodulate a desired signal. It then determines a signal to noise ratio (SNR) of the received signal using SNR estimation. The BTS then compares the determined SNR to the nominal power level to generate a power control bit. Typically power control is only concerned with a single bit wherein a 1 indicates to the mobile unit to increase transmission power and a 0 indicates to the mobile unit to decrease transmission power. The power control bit is multiplexed with data and transmitted to the mobile unit as a Transmit Power Control (TPC). The mobile unit receives the TPC signal, demodulates it, and separates the PCB (demultiplexes it). The mobile unit then converts the PCB to a power transmission gain (positive or negative) of the output power in the Extract Power Control Bit and Convert to Power Step Size block. This is considered closed loop power control.
There are also methods in 3GPP considered outer loop power control. In outer loop power control systems, the BTS adjusts the nominal power level based upon a Frame Error Rate (FER) probability for a particular nominal power level. The BTS measures the FER probability and determines a SNR threshold. If at the output of the Viterbi decoder, the FER is high, the nominal power level is increased. If the FER is low, the nominal power level is decreased.
Channel fading without power control leads to a standard deviation of 5.5 dB for all fading frequencies. However, due to the deep fades of the desired signal, the standard deviation may decrease more than 20 dB with respect to the required signal level. This leads to the increase of the error probability for a particular mobile unit.
Based on the current 3GPP specification, closed loop power control results in a considerable reduction of SNR deviation for small fading frequencies (e.g. in the range of 8-15 Hz). However, the efficiency is greatly decreased when the fading frequency is above 30 Hz. This is because of the conventional fixed power control steps of 0.25 dB, 0.5 dB and/or 1 dB and the delay (at least one slot of power control bit command) which are not able to track the changes of the signal power in the channel in fast fading environments. For the same reasons large power overshooting (i.e. too many increases or decreases to the transmission power in the mobile unit) occurs at the BTS input for all fading frequencies.
Conventional 3GPP systems operate with a fixed power control step size (e.g. 1 dB) and a fixed power control command transmission delay. When a signal from a mobile unit experiences a deep fade, the BTS sends consecutive power increasing commands to the mobile unit. The mobile unit receives these commands and increases its transmission power to compensate for the deep fading. However the mobile unit continues to increase its power even after the deep fade period ends, due to the power control command transmission delay. This continued increase in power causes the power overshoot (see FIG. 2). Power overshooting negatively impacts the uplink power control performance by increasing the standard deviation of power control for the particular mobile unit and increasing the over all interference experienced by transmissions to the BTS. Power overshooting has been observed to be as high as 5 dB and has been known to occur at all fading frequencies.
Accordingly, there exists a need for a power control system which minimizes power overshoot.
There also exists a need for a power control system which maximizes uplink capacity.
There exists the need for a power control system which minimizes the standard deviation from the nominal power level.
Accordingly, it is an object of the present invention to provide a power control system which minimizes power overshoot.
It is another object of the invention to provide a power control system which minimizes the standard deviation from the nominal power level.
It is still another object of the invention to provide a power control system which employs a variable step size.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.